linux-kernel - Re: [PATCH-v5 RESEND 4/5] Documentation: binding: add sclk adjustment properties to i2c-pxa

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Message-ID: <20151025083748.GC2159@katana>
Date:	Sun, 25 Oct 2015 09:37:48 +0100
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Vaibhav Hiremath <vaibhav.hiremath@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, robh+dt@...nel.org,
	robert.jarzmik@...e.fr, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH-v5 RESEND 4/5] Documentation: binding: add sclk
 adjustment properties to i2c-pxa

On Mon, Aug 24, 2015 at 11:29:37AM +0530, Vaibhav Hiremath wrote:
> With addition of PXA910 family of devices, the TWSI module supports
> new feature which allows us to adjust SCLK. i2c-pxa driver takes input
> configuration in nsec and converts it to respective bit-fields,
> 
>  - i2c-sclk-low-time-ns : SCLK low time (tlow)
>    This property is used along with mode selection.
>  - i2c-sclk-high-time-ns : SCLK high time (thigh)
>  - i2c-start-hold-time-ns : Used in case of high speed mode for start bit
>    hold/setup wait counter.
>  - i2c-stop-hold-time-ns : Used in case of high speed mode for stop bit
>    hold/setup wait counter.
>  - i2c-sda-hold-time-ns : Used to calculate hold/setup wait counter for
>    standard and fast mode.
> 
> Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@...aro.org>

Ooookay, after checking some datasheets, I finally stumbled over the
fact that this driver is not using "clock-frequency" to determine the
bus speed. However, this is the standard way of defining that in DT, so
this driver should adhere to that, too, and not invent something new.
Previously, I was under the impression that the above parameters were
needed for fine-tuning and couldn't fully grasp why. But this is for
defining the bus speed which is a no-go.

It looks to me that older PXA only have a bit to select between 100 and
400kHz while this newer one can have arbitrary frequencies. Correct?

So, what the driver should do:

- Keep support for the old "mrvl,i2c-fast-mode" binding which should be
  marked as deprecated

- get the value of the standard property "clock-frequency"

- for old PXAs, complain if not 100 or 400kHz, otherwise set the bit
  accordingly

- for new PXAs, calculate the ilcr and iwcr values from
  "clock-frequency"

Makes sense?

Regards,

   Wolfram


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